To appear in: A. Wilson (ed) Proc. 10th European Solar Physics Meeting, Prague, ESA SP-506, in press (2002)
DOPPLER OSCILLATIONS IN HOT CORONAL LOOPS
W. Curdt 1), T.J. Wang 1), D.E. Innes 1), S.K. Solanki 1), I.E. Dammasch 1), B. Kliem 2), L. Ofman 3)
1)
Max-Planck-Institut für Aeronomie; Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
2)
Astrophysikalisches Institut Potsdam; An der Sternwarte 16, 14482 Potsdam, Germany
3)
NASA Goddard Dept. of Physics, Catholic University, and Space Flight Center, USA
tel: +49 5556 979 420 / fax: +49 5556 979 240
e-mail: curdt@linmpi.mpg.de
ABSTRACT
Recently, a new kind of damped oscillations of hot
coronal loops was revealed by the Solar Ultraviolet
Measurements of Emitted
Radiation (SUMER)
spectrometer on SOHO. Such events seem to be a
common feature observed in active region loops, seen
very often when these lines brighten. The oscillations
always have an impulsive trigger and are strongly
damped while they cool down. However, in lines
formed at coronal temperatures of 2 MK never any
signature of these oscillations has been observed. In
this study, we present the main properties of Doppler
oscillations derived from a statistical study of 17 flarelike events, and a comparison with TRACE transverse
loop oscillations. We also discuss the oscillation modes
and their damping mechanism.
time range comparable to that of the TRACE loops, but
oscillation periods distinctly longer than those observed
by TRACE. This difference is confirmed by new
observations with a higher cadence reported in this
study. We present a representative case and statistical
results of the hot loop oscillations analyzed so far.
OBSERVATIONS
In all cases, the SUMER spectrometer slit was placed
at a fixed position in the corona about 100” above an
active region at the limb. The earlier observations
during 1999 - 2001 were made in a wide spectral
window with a 162 s exposure time. The new
observations obtained in April 2002 were made in three
lines (Si III / S III, Ca X, and Fe XIX) with a high
cadence of 50 s. The selected lines cover a wide
temperature range from T = 104 to 107 K.
INTRODUCTION
Transverse oscillations of active region loops were first
discovered with TRACE in EUV bands (Aschwanden
et a1. 1999), and more cases have been reported in a
recent statistical study by Schrijver et al. (2002). These
transverse loop oscillations are excited by flares, and
show a very quick decay on time scales from 3 to 21
min,
with oscillation periods of 2 to 11 min.
Aschwanden et al. (1999) and Nakariakov et al. (1999)
have interpreted the loop oscillation as a fundamental
resonance of the kink mode, and Nakariakov et al.
(1999) concluded that the rapid damping is due to
anomalously high viscosity, or resistivity in the coronal
plasma. Ofman et al. (2002) investigated the scaling of
the oscillation damping time for the TRACE loops, and
found that the scaling agrees best with phase mixing as
the damping mechanism.
Recently, additional evidence of damped oscillating
coronal loops was found by SUMER spectral
observations (Kliem et al. 2002, Wang et al. 2002a,
2002b, 2002c). They show that Doppler shift oscillations in hot coronal loops (T 107 K) have a damping
Figure 1: A recurring impulsive flare-like event
observed in the Fe XIX λ 1118 flare line both in
radiance and in Doppler flow. Oscillations ocurred at
the place where the slit crossed a SXT loop, indicative
of hot plasma dynamics. No signatures are seen in the
lines at T < 2 MK.
EXAMPLE
We have selected a text book oscillation event observed
on Sep 29, 2000 representative of many more cases for
further analysis. The Fe XIX radiance and Doppler flow
during this event is shown in Fig. 1. Fitting the average
time profiles of the Doppler oscillation by a function
v(t) = v0 + vm sin ( ωt + φ ) e-λt
(1)
yields oscillations periods of 25 – 30 min, with a decay
time of 21 – 29 min. Brightness variations show no
fluctuation behaviour with the oscillation period.
Based on an ellipse model applied on SXT images, we
derive geometrical parameters of the oscillating loop:
the loop length L = 473 Mm, the loop height (radial
distance to solar surface) H = 104 Mm, the azimuth
angle to EW of 23 deg, the inclination angle to the
vertical plane of 28 deg, the angle between loop
baseline and line-of-sight of 34 deg.
The initial red shifts suggest the trigger may be hot
plasma flow injected into the loop from one footpoint,
seen as a brightening in SXT. With a sound speed of
Cs  416 km/s for T = 6.3 MK (Fe XIX ), we obtain a
period of standing slow waves, P  2L/Cs = 38 min,
this is about 1.4 times the observed periods.
With B = 10 G and n = 1.0109 /cm3, we obtain CA =
689 km/s, Ck = 930 km/s, and a period of global kink
mode P = 2L/Ck = 17 min, and a plasma  of 0.44 in
the coronal loop.
STATISTICAL RESULTS
So far we have analyzed 35 Doppler oscillation
components in 17 flux enhancement events of hot
plasma (>6 MK). In each event, we identified several
oscillation components along the slit, due to differences
in Doppler shift, intensity, or line width. All events
show impulsive flux variations typical for flares, but
only 3 of the 17 events were associated with GOES
flares above detection limit. A comparison of the
statistical results from TRACE and SUMER is given in
Figure 2, where we show histograms of the oscillation
periods, the damping times, the Doppler flows, and the
maximum displacements observed by both instruments.
In the SUMER case, the displacement amplitude has
been derived by integration of equation (1). Although
the new observations had a high cadence of 50 s, which
would allow detection of short-period oscillations
down to periods of 3 minutes, no oscillations are found
with periods shorter than 10 min. We measure
oscillation periods ranging from 10.8 to 31.1 min with
a mean of (18.5  5.8) min, damping times ranging
from 5.5 to 28.9 min with a mean of (14.6  6.3) min,
maximum Doppler velocities ranging from 14 to 315
km s-1 with a mean of (102  81) km s-1 and derived
maximum displacements ranging from 2 to 54 Mm
with a mean of (18  15) Mm.
If we compare the parameter distributions of the
SUMER oscillation events to the TRACE transverse
oscillations events triggered by strong flares
(Aschwanden et al. 2002), we find that:
-
the Doppler oscillation periods found by SUMER
are distinctly longer than those observed by
TRACE. Standing slow modes roughly match the
observed periods.
-
the damping time found by SUMER is comparable
to the TRACE damping time.
-
the maximum velocities and displacement
amplitudes found by SUMER are larger than
TRACE values.
We also investigated the scaling law between damping
time and oscillation periods observed by both
instruments, (c.f. Figure 3) and found significant
differences.
MHD SIMULATION: SLOW WAVE DAMPING
BY THERMAL CONDUCTION
Using 1D MHD code, Ofman and Wang (2002) model
the oscillations and damping of slow magnetosonic
waves in a model coronal loop. They find that:
· Due to the high temperature of the loops, the large
thermal conduction, which depends on temperature as
T2.5, leads to rapid damping of the slow waves on a
time scale comparable to observations.
· The scaling of the dissipation time with period agrees
well with the 35 cases observed by SUMER
· The decay time due to compressive viscosity alone is
an order of magnitude longer than the observed decay
times.
Figure 2: Comparison of the oscillation periods, the damping constants, the Doppler flows, and the maximum
displacements observed by SUMER and TRACE (TRACE parameters from Aschwanden et al. 2002).
We have used the following parameter set for our 1D MHD simulations:
ted SXT observations indicate that they are hot loop
oscillations.
loop length:
L = 0.57 Rsun = 400 Mm,
density:
n0 = 5108 /cm3 ,
temperature:
T = 6.3 or 8 MK
the corresponding sound speeds are
A statistic study on 35 oscillations in 17 flare-like
events shows that these Doppler oscillations have
periods of 11 to 31 min, with decay times of 6 to 29
min, and show an initial large shift pulse with peak
velocities up to 200 km/s. Unlike TRACE loop
transverse oscillations that were mostly triggered by
strong flares, the SUMER oscillation events happened
mostly without associated GOES flares, suggesting a
different trigger mechanism.
cs = 416 or 469 km/s
and the initial disturbance velocity profile is
v(x) = v0 sin( 2 x / L) with
v0/cs = 0.18
Our results are shown in Figure 4.
CONCLUSION
SUMER spectral observations have revealed a new
kind of coronal plasma oscillations, that were only
detected in hot flare lines of T > 6 MK. The coordina-
Doppler oscillations have a distinctly longer period
than those observed by TRACE. We find that slow
standing waves roughly match the observed periods,
while to produce similar periods by a global kink mode
would imply an unusual environment   1 in hot
coronal loops. Moreover, a nonlinear, dissipative MHD
model also strongly suggests that the observed Doppler
oscillations are signatures of damped slow
magnetosonic waves.
Vx(t) at x=0.2 R_sun,
perturbed density, n(t)
perturbed temperature, T(t)
Figure 3: Scaling law between damping time and
oscillation period found by TRACE and SUMER.
Figure 4: Results from the MHD simulation
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